Recently, along with advancements toward higher integration and higher speed of semiconductor elements, a finer pattern rule has been required. In this situation, various techniques have been developed in regard to how patterning process can be performed more finely and precisely on light sources used in lithography with light exposure, which is a commonly-employed technique at present.
As the light source for lithography employed in resist pattern formation, light exposure using a g-beam (436 nm) or an i-beam (365 nm) of a mercury lamp is widely used for portions where the degree of integration is low. Meanwhile, for portions where the degree of integration is high and finer patterning is required, lithography using a KrF excimer laser (248 nm) or an ArF excimer laser (193 nm) with shorter wavelengths has also been actually used. Moreover, for the most advanced generation requiring further finer patterning, lithography with extreme ultraviolet ray (EUV, 13.5 nm) is about to be put into practical use.
As the thinning of resist patterns progresses as described above, a monolayer resist method, which is employed as a typical resist patterning process, becomes well known to increase the ratio of a pattern height to a pattern line width (aspect ratio), so that pattern collapse occurs due to the surface tension of a developer during development. Against this background, a multilayer resist method in which a pattern is formed by laminating films having different dry etching properties has been known to be excellent in forming a pattern with a high aspect ratio on a stepped substrate. There have been developed: a 2-layer resist method in which a photoresist layer made of a silicon-containing photosensitive polymer is combined with an underlayer made of an organic polymer containing carbon, hydrogen, and oxygen as main constituent elements, for example, a novolak polymer (Patent Document 1); and a 3-layer resist method in which a photoresist layer made of an organic photosensitive polymer used in a monolayer resist method is combined with a middle layer made of a silicon-based polymer or a silicon-based CVD film, and an underlayer made of an organic polymer (Patent Document 2).
In this 3-layer resist method, first, a fluorocarbon-based dry etching gas is used to transfer the pattern of the photoresist layer to the silicon-containing middle layer. Then, using the pattern as a mask, dry etching with an oxygen-containing gas is performed to transfer the pattern to the organic underlayer film containing carbon and hydrogen as main constituent elements. The resultant is used as a mask to form the pattern on a substrate to be processed by dry etching. However, in semiconductor element manufacturing processes after the 20-nm generation, when such an organic underlayer film pattern is used as a hard mask to transfer the pattern to a substrate to be processed by dry etching, phenomena are observed in which the underlayer film pattern is twisted and/or curved.
The carbon hard mask formed immediately above the substrate to be processed is generally an amorphous carbon (hereinafter CVD-C) film prepared by a CVD method from a methane gas, an ethane gas, an acetylene gas, and the like as raw materials. The amount of a hydrogen atom in the CVD-C film can be reduced quite small, and this film is known to be very effective against the twisting and curving of the pattern as described above. Nevertheless, it is also known that when the substrate to be processed used as a base has a step, it is difficult to fill such a step into a flat state due to the characteristics of the CVD process. As a result, when a substrate to be processed having a step is coated with a CVD-C film and then patterned with a photoresist, the step of the substrate to be processed causes the applied surface of the photoresist to have a step. This makes the photoresist film thickness non-uniform, and consequently the focus margin and the pattern profile during lithography deteriorate.
On the other hand, it is known that when the underlayer film serving as the carbon hard mask formed immediately above the substrate to be processed is formed by a spin coating method, there is an advantage that a step(s) of the stepped substrate can be filled into a flat state. Planarizing the substrate using the underlayer film composition reduces fluctuation in film thickness of a silicon-containing middle layer and a photoresist coated thereon, can increase the focus margin in lithography and can form a correct pattern.
Hence, there are demands for: an underlayer film composition which allows formation of an organic underlayer film by a spin coating method, the organic underlayer film enabling formation of a film having high etching resistance in dry etching a substrate to be processed and high planarizing property on the substrate to be processed; and a method for forming such an underlayer film.